(a) Field of the invention:
The present invention concerns pn junction semiconductor devices using Group II-VI compound semiconductors and their manufacturing method, and more particularly it pertains to ZnSe pn junction devices and their manufacturing method.
(b) Description of the prior art:
Research and development of pn-junction light-emitting diodes which are so-called LEDs using various kinds of semiconductor materials are under way at present.
There have been and are being manufactured various different kinds of light-emitting devices having different light emission wavelengths to comply with various different purposes. The wavelengths of the light range roughly from the infrared region to the color green in the visible region. Some examples are as follows.
LEDs made of GaAs having an energy band gap of about 1.43 eV emit light in the infrared region at a peak intensity of about 9100 .ANG.. GaP LEDs which are not doped and which have an energy band gap of 2.26 eV emit light of green color having its peak intensity at 5550 .ANG., while GaP LEDs which are doped with nitrogen as an impurity emit a yellow-color light having its peak intensity at about 5800 .ANG.. Also, in mixed crystals Ga.sub.1-x Al.sub.x As, it is known that, by varying the component ratio x, LEDs emitting a red color light having a peak intensity at about 6500 .ANG. or a yellow color light having a peak intensity of about 5900 .ANG. are obtained.
The wavelength for the peak intensity of light which is emitted from an LED depends strongly on the energy band gap of the semiconductor material with which the LED is made.
The wavelength for light emission at peak intensity appears in a wavelength region longer than the absorption edge of the semiconductor. The wavelength .lambda. of the absorption edge, which is the shortest wavelength possible for emission, is expressed by: ##EQU1## wherein: h represents Planck's constant;
c represents the velocity of light; and PA1 E.sub.g represents the energy band gap of the semiconductor material, expressed by eV.
The wavelengths of emitting light of known LEDs extend up to about 5550 .ANG. which is green color. And, there have been provided no LEDs whose emitting light has a higher energy than that mentioned above, i.e. a light of a shorter wavelength which, in terms of color, is in the region of blue-green, blue and violet, although there are strong demands for such LEDs. Until now, however, no such LEDs have been obtained because of the reasons described below.
In order to obtain an LED having such a region of wavelength of emitting light as mentioned above, it is necessary to use semiconductor materials having an energy band gap wider than that of Group III-V compound semiconductors such as GaAs, GaP or Ga.sub.1-x Al.sub.x As. As such semiconductors, there are Group II-VI compound semiconductors such as ZnSe (E.sub.g .apprxeq.2.8 eV) and ZnS (E.sub.g .apprxeq.3.6 eV). These semiconductors have wide energy band gaps, so that they have attracted the interest of researchers and have been studied. Nevertheless, owing to various technical problems, they have not been put to practice yet. The Group II-VI compound semiconductors such as ZnSe stated above have a wide energy band gap, and for a long time crystals have been produced either as a photoconductive semiconductor or as an electroluminescent semiconductor. There have been practiced various kinds of methods of producing these semiconductor crystals. In order to obtain a single crystal, however, which is large enough to permit the fabrication of a semiconductor device, there has been adopted a melt growth method as represented by the Bridgman method. As is well known, however, there has been technical difficulties in achieving free control of the conductivity types of Group II-VI compound semiconductors. In Table 1 are shown the conductivity types and energy band gaps of Group II-VI compound semiconductors which have been obtained in the past.
TABLE 1 ______________________________________ ZnS ZnSe ZnTe CdS CdSe CdTe ______________________________________ Conductivity n n p n n n type p Energy band 3.6 eV 2.8 eV 2.2 eV 2.5 eV 1.74 eV 1.5 eV gap ______________________________________
The semiconductor materials listed in Table 1 invariably have a wide energy band gap, and will bring about a very effective result if LEDs are manufactured with them. However, they have not permitted free control of their conductivity type until now. With ZnS, CdS or ZnSe, n-type conductivity is easily obtained. However, even by doping an acceptor impurity in order to obtain the p type, the result would be that the doped crystal still remains an n-type or p-type having a very high resistivity. Even when the crystal happens to the p-type, its control is so difficult that the formation of a pn junction necessary for an LED has not been possible.